Nroug7: That's correct, Realize is the American form of the word realise. which is why spell check marks realise as wrong when it is correct in most other countries. As a general rule though, i use realize when im on the internet.
michaelleung: The problem with American English is that the very idea of having a difference to British English was based on the fact that Noah Webster, who created the first American dictionary, wanted to differentiate his country from Britain. Quite silly, because it only makes Webster seem like a petulant child who wanted to be different from the others.
In many cases, this is the case with American spellings. However, realize is actually the entry used in the Oxford English Dictionary, while realise is an alternate spelling. In actuality, the whole "-ise" over "-ize" spelling is something that migrated over from French. This is probably why it is actually more common in Canada, and not used at all in the US.
http://oxforddictionaries.com/definition/-ize?q=-ize 1 The form -ize has been in use in English since the 16th century; although it is widely used in American English, it is not an Americanism. The alternative spelling -ise (reflecting a French influence) is in common use, especially in British English. It is obligatory in certain cases: first, where it forms part of a larger word element, such as -mise (= sending) in compromise, and -prise (= taking) in surprise; and second, in verbs corresponding to nouns with -s- in the stem, such as advertise and televise.